PROJECT SUMMARY Our long term goal is to better understand how neurotransmitter receptor density is maintained and regulated at mature and developing synapses in vivo. We are using the neuromuscular junction (NMJ), an excitatory cholinergic synapse between motor neurons and muscle fibers as our model system, because of the significant impact on human health of disorders at this synapse and because it provides the most accessible model system to study the process of neurotransmitter receptor recycling in vivo. Until recently receptor recycling was thought to be an exclusive property of neurotransmitter receptors in the central nervous system, but we discovered that significant numbers of acetylcholine receptors (AChRs) are recycled back onto the postsynaptic membrane after internalization, contributing to the maintenance of the postsynaptic density of receptors at mature synapses. We will pursue two goals to assess potential roles for recycling during synaptic development and in maturity and two goals to test the roles of molecules that are strong candidates for participation in the recycling process. We have focused on the roles of alpha-dystrobrevin and alpha-syntrophin in regulating the recycling of AChR onto the postsynaptic membrane because mutations in these proteins are clearly associated with human disorders of neuromuscular transmission, and mutations of these genes in mice have been demonstrated to dramatically alter the number of AChR at mature synapses, as might be expected if receptor recycling was aberrant. Understanding the molecular basis of receptor recycling could lead to more effective intervention and therapy that could increase synaptic transmission by increasing AChR number or density in neuromuscular diseases. Furthermore, it seems likely that some aspects of receptor recycling will be common to all cells, so we expect that these results will be useful not only in understanding the development of the neuromuscular junction, but will provide insights into the long-term role of receptor recycling in synaptogenesis and synaptic plasticity at less accessible central synapses.